1. Field
The present invention relates to a ferromagnetic amorphous alloy ribbon for use in transformer cores, rotational machines, electrical chokes, magnetic sensors and pulse power devices and a method of fabrication of the ribbon.
2. Description of the Related Art
Iron-based amorphous alloy ribbon exhibits excellent soft magnetic properties including low magnetic loss under AC excitation, finding its application in energy efficient magnetic devices such as transformers, motors, generators, energy management devices including pulse power generators and magnetic sensors. In these devices, ferromagnetic materials with high saturation inductions and high thermal stability are preferred. Furthermore, the ease of the materials' manufacturability and their raw material costs are important factors in large scale industrial use. Amorphous Fe—B—Si based alloys meet these requirements. However, the saturation inductions of these amorphous alloys are lower than those of crystalline silicon steels conventionally used in devices such as transformers, resulting in somewhat larger sizes of the amorphous alloy-based devices. Thus efforts have been made to develop amorphous ferromagnetic alloys with higher saturation inductions. One approach is to increase the iron content in the Fe-based amorphous alloys. However, this is not straightforward as the alloys' thermal stability degrades as the Fe content increases. To mitigate this problem, elements such as Sn, S, C and P have been added. For example, U.S. Pat. No. 5,456,770 (the '770 Patent) teaches amorphous Fe—Si—B—C—Sn alloys in which the addition of Sn increases the alloys' formability and their saturation inductions. In U.S. Pat. No. 6,416,879 (the '879 Patent), addition of P in an amorphous Fe—Si—B—C—P system is taught to increase saturation inductions with increased Fe content. However, addition of such elements as Sn, S and C in the Fe—Si—B-based amorphous alloys reduces the ductility of the cast ribbon rendering it difficult to fabricate a wide ribbon, and addition of P in the Fe—Si—B—C-based alloys as taught in the '879 Patent results in loss of long-term thermal stability which in turn leads to increase of magnetic core loss by several tens of percentage within several years. Thus, the amorphous alloys taught in the '770 and '879 Patents have not been practically fabricated by casting from their molten states.
In addition to a high saturation induction needed in magnetic devices such as transformers, inductors and the like, a high B—H squareness ratio and low coercivity, Hc, are desirable with B and H being magnetic induction and exciting magnetic field, respectively. The reason for this is: such magnetic materials have a high degree of magnetic softness, meaning ease of magnetization. This leads to low magnetic losses in the magnetic devices using these magnetic materials. Realizing these factors, the present inventors found that these required magnetic properties in addition to high ribbon-ductility were achieved by maintaining the C precipitation layer on ribbon surface at a certain thickness by selecting the ratio of Si:C at certain levels in an amorphous Fe—Si—B—C system as described in U.S. Pat. No. 7,425,239. Furthermore, in Japanese Kokai Patent No. 2009052064, a high saturation induction amorphous alloy ribbon is provided, which shows improved thermal stability of up to 150 years at 150° C. device operation by controlling the C precipitation layer height with addition of Cr and Mn into the alloy system. However, the fabricated ribbon exhibited a number of protrusions on the ribbon surface facing the moving chill body surface. A typical example of protrusion is shown in FIG. 1. The basic arrangement of casting nozzle, chill body surface on a rotating wheel and resulting cast ribbon is illustrated in U.S. Pat. No. 4,142,571.
Upon careful analysis of the nature of the protrusion and its formation, it was found that ribbon “packing factor” (PF) decreased when the height of a protrusion exceeded four times the ribbon thickness and/or when the number of protrusions exceeded 10 per 1.5 m along the ribbon's length direction. Here, packing factor, PF, is defined by the effective volume of ribbon when the ribbon is stacked or laminated. A higher PF is desired when a stacked or laminated product is used in a magnetic component when a smaller magnetic component is needed.
Thus, there is a need for a ferromagnetic amorphous alloy ribbon which exhibits a high saturation induction, a low magnetic loss, a high B—H squareness ratio, high mechanical ductility, high long-term thermal stability, and reduced number of ribbon surface protrusions with high level of ribbon fabricability, which is an objective of the present invention. More specifically, a thorough study of the cast ribbon surface quality during casting led to the following findings: when protrusion height exceeded four times the ribbon thickness or when the number of protrusions exceeded 10 over cast ribbon length of 1.5 m, casting had to be terminated in order to meet a packing factor PF>82% which was a minimum PF required in the industry. Generally protrusion height and number increased with casting time. For conventional amorphous alloy ribbons having saturation induction, Bs, less than 1.6 T, ribbon casting time was about 500 minutes before protrusion height exceeded four times the ribbon thickness or protrusion number increased to 10 per 1.5 m length of cast ribbon. For the amorphous alloy ribbons having Bs>1.6 T, casting time was often reduced to about 120 minutes, resulting in cast termination rate of 25%. Thus, it is clearly needed to clarify the cause of protrusion formation and to control it, which is another aspect of the present invention.